Ciliary neurotrophic factor (CNTF) is well known as a growth/survival factor of neuronal tissue. We investigated the expression of CNTF and its specific receptor alpha (CNTFRα) in a unilateral ureteral obstruction (UUO) model. Complete UUO was produced by left ureteral ligation in Sprague-Dawley rats. The animals were sacrificed on days 1, 3, 5, 7, 14, 21, and 28 after UUO. The kidneys were fixed, and processed for both immunohistochemistry and in situ hybridization. CNTF immunoreactivity in sham-operated kidneys was observed only in the descending thin limb (DTL) of the loop of Henle. In UUO kidneys, CNTF expression was induced in the S3 segment (S3s) of the proximal tubule from day 1, and progressively expanded into the entire S3s and a part of the convoluted proximal tubules, distal tubules (DT), and glomerular parietal epithelium up to day 7. Upregulated CNTF expression was maintained to day 28. From day 14, the inner medullary collecting duct showed weak CNTF immunoreactivity. The CNTFRα mRNA hybridization signal in sham-operated kidneys was weakly detected in the DTL, DT, medullary thick ascending limb, and in a few S3s cells. After UUO, CNTFRα mRNA expression increased progressively in both the renal cortex and the medulla up to day 7 and increased expression was maintained until day 28. The results suggest that the S3s may be the principal induction site for CNTF in response to renal injury, and that CNTF may play a role in chronic renal injury.